1. Field of Invention
The present invention is related to producing welds with reduced residual stress level and without commonly known defects. More particularly invention relates to an improvement in the process for prevention of build up of residual stress disclosed in applicant's U.S. Pat. No. 4,386,727, Jun. 7, 1983.
The process described in applicant's patent mentioned above has received acclaim from scientific circles. But improvements are necessary and possible on the basis of further understanding of the welding process itself, the manner in which structures of microstructure are formed and causes how stresses come into effect in a weld.
2. Background
It is obvious that molten metal solidifies to form a weld nugget and it also makes joint/s between the interface of liquid metal and the solid face of the cup formed in base metals. Welding itself is a quasi-stationary state, in which a hill of heat moves over stationary weld plates arranged in juxta position for welding. Such a hill of heat is with set physical dimensions once welding attains a stable condition, as first observed by Rosenthal in 30's, and it moves along the center line of the weld at the welding speed.
As disclosed in my U.S. Pat. No. 4,386,727 the external stress of low value, applied at low frequency at center of weld nugget at one end, in experimental welds, prevents build up of residual stress during solidification. The stress required is of low value, because bond strength at solidification temperatures is low. Process in U.S. Pat. No. 3,741,820 and its improvement, U.S. Pat. No. 4,968,359, attempts application of frequencies close to natural mechanical resonant harmonic or its subharmonic frequencies of the entire weldment after weld solidification. These generate high stresses. A weld usually occupies only a very small fraction of volume of a weldment. Therefore major portion of energy developed is used in larger volume of the weldment; and only marginal benefits may be achieved in reducing residual stress in solidified weld. Further, such stresses do not act on individual crystals, but on solidified (monolithic) mass of crystals which is the weld nugget, along with other part of the weldment. Further the microstructure in the nugget is then at a temperature far lower than the solidification temperature and therefore bond strengths are high. These facts cause only marginal success in attempts to reduce residual stress in welds. Results of such processes are normally compared with the widely used thermal stress relief process which gives better results in reduction of residual stress. Another valid objection is that in such processes the useful fatigue life of the weldment is used up and therefore reduced.
In the process disclosed here, stresses are applied at the solidification temperature, to the Trailing Edge of the Weld (TEW), in particular to solid surface resulting from preceding Solidifying Metal Zone in the Second order Quasi Stationary (SQS) state; further as bond strengths are lower, input of energy at lower stress amplitudes is necessary. Further, stress applied is transmitted to individual growing crystals in the Liquid Metal Zone (LMZ) through semi-liquid state, which has a certain degree of mobility. These conditions should be compared with conditions when only a fraction of the applied stress that may be reaching the solidified mass of the weld when stress frequencies closer to harmonic or sub-harmonic are applied to the entire weldment. Minor positive effects on welds are observed by application of such processes during welding. These can be inferred from results which are embodiment of U.S. Pat. No. 4,386,727.
Modification of the microstructure and therefore reduction of residual stress and elimination of the discontinuities such as porosity, are distinct benefits of the process disclosed herein; therefore, the instant invention is a necessary improvement over all the methods, known so far.